1. Field of the Invention
The present invention relates to an optical memory disc, to which a laser beam is impinged to record, read out or erase information onto or from the optical memory disc.
2. Description of the Prior Art
Recently, an optical memory system has been developed, which optically stores information in high density and in mass storage order. In the optical memory system of the add-on-memory type or the erasable memory type, guide signals or guide addresses are normally recorded on the optical memory disc along concentric circular guide grooves or a spiral guide groove so as to control the optical beam position. In order to ensure an accurate operation, the concentricity of the circular guide grooves (spiral groove) and a rotation center of the optical memory disc must be maintained at a high accuracy. If the rotation center of the optical memory disc is displaced from the center of the circular guide grooves (spiral groove), the tracking operation becomes difficult because of the whirling.
In the conventional optical memory disc, a circular opening is formed in the optical memory disc at the center thereof, and the optical memory disc is driven to rotate while the circular opening is coupled to the spindle of a turntable of an optical memory system. The concentricity adjustment is difficult to control because the circular opening is formed directly in the optical memory disc. Furthermore, there is a possibility that the optical memory disc is damaged when the circular opening is formed in the substrate of the optical memory disc.